The organisms referred to as protozoans (“first animals”) constitute of diverse group of eukaryotic (mostly) unicellular organisms. In protozoans all life functions are carried out within the confines of a single cell. Although there are obviously no organs or tissues in protozoans, they are far from “simple” organisms as they are sometimes described. In fact, the cells of some species show the greatest complexity and internal organization of any organisms on Earth!

General protozoan characteristics include: small size, unicellular (but some species are colonial or have multicellular stages), body naked or covered by an exoskeleton (test) formed of silica or calcium carbonate. With over 64,000 living species, protozoans show a fantastic diversity of forms. Although they are found wherever life exists, protozoans always require moisture, which restricts them to a narrow range of environmental conditions in fresh water or marine habitats, the soil, decaying organic matter or inside the bodies of plants and animals. Many forms are ecologically important, forming essential links in food chains and decomposer systems.

About 10,000 species have close (symbiotic) relationships with animals or plants. These relationships may be mutualistic (both partners benefit), commensalistic (one benefits, while the other is neither helped nor harmed) or parasitic (the parasite benefits; the host is harmed). In fact, some of the most important diseases of humans and domestic animals are caused by parasitic protozoans!